Field of the Invention
The present invention relates to a display device for forming an optical image thereon by modulating an incident light ray, and in particular to a display system for magnifying and projecting the image formed on the display device (this display system referred to as "LCD image projection type television system" hereinafter), and a display system for displaying a video image in recording by means of a video camera or the like (this display system referred to as "view finder" hereinafter).
Description of the Prior Art
A display device employing a liquid crystal display panel (referred to as an "LCD panel" hereinafter) has been enthusiastically studied and developed because of its capability for light weight and thin configuration designing. In recent years, pocket television sets employing a twisted nematic (TN) mode LCD panel utilizing rotatory polarization property of the LCD panel for image display has been put into practical use. Furthermore, LCD image projection type television systems, view finders, and the like using the above-mentioned LCD panel as a display device have been also put in practical use.
FIG. 23 is a diagram showing an equivalent circuit of a conventional display device employing an active matrix type LCD panel. Referring to FIG. 23, reference characters G.sub.1 through G.sub.m denote gate signal lines each having its one end connected to a gate drive IC 231. The gate drive IC 231 outputs a voltage for turning on (this voltage referred to as "on-voltage" hereinafter) a thin film transistor. (referred to as "TFT" hereinafter) serving as a switching element or a voltage for turning off (this voltage referred to as "off-voltage" hereinafter) the TFT. Meanwhile, reference characters S.sub.1 through S.sub.n denote source signal lines each having its one end connected to a source drive IC 232.
The TFT is each represented by T.sub.ij (i: an integer belonging to a row of the matrix, j: an integer belonging to a column of the matrix) which is connected to a pixel electrode P.sub.ij. TN liquid crystals (not shown) are interposed between the pixel electrode P.sub.ij and an opposite electrode (not shown). The TN liquid crystals change their orientation conditions according to a voltage input to the pixel electrode thereby to modulate incident light.
The following describes a drive circuit for the conventional display device. FIG. 25 shows a block diagram of a drive circuit for the conventional display device.
Referring to FIG. 25, reference numeral 251 denotes an amplifier for amplifying a video signal to a specified value, while reference numeral 252 denotes a phase division circuit for generating a video signal of positive polarity and negative polarity. It is herein noted that the positive polarity means an electric potential higher than the electric potential at the opposite electrode (referred to as the "common voltage" hereinafter) while the negative polarity means an electric potential lower than the electric potential at the opposite electrode. Reference numeral 253 denotes an output changeover circuit for outputting an AC video signal which is inverted in polarity every one field (1F) or every one horizontal scanning line (1H). Reference numeral 254 denotes an LCD panel, and reference numeral 255 denotes a drive IC control circuit for effecting synchronization and control of the source drive IC 232 and the gate drive IC 231.
The following describes the operations of the drive circuit shown in FIG. 25 of the conventional display device. First in the amplifier 251, a gain adjustment is effected so that the amplitude of a video signal corresponds to the electric and optical characteristics of the liquid crystals. Then the video signal which has undergone the gain adjustment is input to the phase division circuit 252 to yield two output video signals, one in positive polarity and the other in negative polarity with respect to the common voltage of the opposite electrode. The video signals of positive and negative polarities are input to the output changeover circuit 253, and the output changeover circuit 253 outputs a video signal which is inverted in polarity every one field (1F) or every one horizontal scanning cycle (1H). The reason why the video signal is inverted in polarity is to apply an AC voltage to the liquid crystals because the liquid crystals are decomposed to be deteriorated when receiving a DC voltage. Then a video signal from the output changeover circuit 253 is input to the source drive IC 232, and the source drive IC 232 applies a sampled video signal to the source signal line of the LCD panel 254 in synchronization with the gate drive IC 231 according to a control signal transmitted from the drive IC control circuit 255.
FIG. 26 is a graph showing a relation between an application voltage V applied to liquid crystals and a light transmission quantity T. It is noted here that the application voltage to the liquid crystals is an AC voltage. Therefore, the application voltage V shown in the graph of FIG. 26 represents the effective value of the AC application voltage V. The light transmission quantity T varies when the application voltage V to the liquid crystals is not lower than V.sub.r, while the light transmission quantity T saturates when the application voltage is at a voltage Vm. Normally, the critical voltage V.sub.r is 1.5 to 1.8 V, while the saturation voltage Vm is about 5.0 to 6.0 V when TN liquid crystals are used.
The following describes the operations of the conventional display device with reference to FIG. 23. For simplifying the description, it is assumed that an output voltage of V.sub.+ or V.sub.- is output from the source drive IC 232 to all the source signal lines S.sub.j (j: 1, ..., n). Basically in the present specification, superimposition of an additional signal + onto the signal voltage V means a signal of a positive polarity with respect to the common voltage, while superimposition of an additional signal--onto the signal voltage V means a signal of a negative polarity with respect to the common voltage. It is further noted that an arbitrary pixel row of a matrix is indicated with a supplementary character i (i: 1, ..., m) attached, and an arbitrary pixel column of the matrix is indicated with a supplementary character j attached.
First, an on-voltage is applied to a gate signal line G.sub.1 from the gate drive IC 231, and an off-voltage is applied to the other gate signal lines G.sub.2 through G.sub.m. In the above case, each T.sub.1j of the TFTs on the first row is turned on so that the signal V.sub.+ output on a source signal line S.sub.j applied from the source drive IC 232 is input to each pixel electrode P.sub.1j arranged on the first row. The liquid crystals on the pixel electrode P.sub.1j changes their orientation conditions to modulate incident light due to the input voltage signal V.sub.+.
Next, the gate drive IC 231 applies an on-voltage to a second gate signal line G.sub.2, while an off-voltage is applied to the other gate signal lines. Then, each TFT T.sub.2j on the second row is turned on so that the signal V.sub.- output on the source signal line S.sub.j is applied from the source drive IC 232 to a pixel electrode P.sub.2j arranged on the second row. In a manner as described above, the gate drive IC 231 sequentially writes the voltages into the pixel electrodes P.sub.ij shifting the on-voltage output position from G.sub.1 to G.sub.m. It is noted here that a time necessary for scanning one gate signal line is referred to as the "1H cycle", and a cycle from the time when a voltage is applied to a pixel electrode to the time when the voltage is subsequently applied to the same pixel electrode is referred to as the "one field cycle" (1F in short) hereinafter. Normally, 1F is 1/60 second. It is further noted that two fields forms one frame, and one frame forms one screen in the television signal.
FIG. 24 shows an output waveform 241 of the voltage signal that is applied from the source drive IC 232 to the source signal lines S.sub.j. The polarity of the output signal is altered every horizontal scanning cycle 1H. The above-mentioned method of driving the display device with the output signal from the source drive IC 232, where the polarity of the voltage signal is altered every 1H cycle is referred to as the "1H inversion drive" hereinafter. When a signal as shown in FIG. 24 is applied to the display device, a horizontal stripe-shaped image is displayed on the LCD panel.
A positive voltage V (P) and a negative voltage V (M) are supplied to the source drive IC 232, and a signal having a voltage ranging from +V.sub.m to -V.sub.m is output within the aforementioned voltage range to drive the LCD panel. It is noted that the central value V.sub.0 of the signal applied to each pixel shown in FIG. 24 preferably coincides with the common voltage. However, the central value V.sub.0 is usually shifted negative in polarity with respect to the common voltage due to a parasitic capacitance between the TFT array gate signal lines and the pixel electrodes, electromagnetic field exerted from the gate signal lines to the opposite electrode, and the like. It is noted here that the central value V.sub.0 of the signal has the same potential as the common voltage, and inputting of the voltage V.sub.0 to each pixel electrode means that the same voltage as the common voltage is input to each electrode for the purpose to provide a simplified description. In the case of inputting the common voltage value V.sub.0, no voltage is applied to the liquid crystals on the pixel electrode P.sub.ij, which causes no change in the orientation of the liquid crystals.
An exemplified LCD image projection type television system using such a conventional display device as a light valve is disclosed, for example, in the Japanese Patent Laid-Open Unexamined Publication No. HEI 2-244089 The above-mentioned example uses as a light valve a display device utilizing the aforementioned twisted nematic (TN) liquid crystals, where a metal halide lamp or a halogen lamp is used as a light source.
The LCD image projection type television system has a following construction. Light emitted from a lamp is separated into three primary color light paths of red, blue, and green (referred to as the "R light component", "B light component", and "G light component" respectively hereinafter) by means of dichroic mirrors. The three primary color light components respectively irradiate three transmission type display devices. The display devices are provided for respectively receiving the R light, B light, and G light components to vary the transmittance of the light according to a video signal to thereby modulate each of the color light components in intensity. The modulated light components are synthesized into an image by a dichroic mirror or mirrors provided on the output side of the display devices so as to be projected on a screen by means of a projection lens.
An exemplified conventional view finder is disclosed, for example, in the Japanese Patent Laid-Open Unexamined Publication No. SHO 62-111233. It is noted here that the term "view finder" is defined to be an object integrating at least a light source such as a light emitting element with a display device.
A rod-shaped fluorescent tube has been conventionally used as a light emitting means. The fluorescent tube has a diameter of 2 to 5 mm when a small size twisted nematic (TN) liquid crystal panel having a display area of about 1 inch is used as a display device. When the TN liquid crystal panel has a display area greater than 1 inch, several fluorescent tubes are used in many cases. Each fluorescent tube emits light forward and backward. In order to utilize the light emitted backward, a concave reflection plate is provided behind the fluorescent tube. The light emitted backward from the fluorescent tube is reflected forward on the reflection plate. A diffusion plate is provided between the fluorescent tube and the TN LCD panel. The diffusion plate is used to diffuse the light from the fluorescent tube to form a surface light source. Light from the surface light source formed by the diffusion plate is incident on the TN LCD panel. The surface light source has an area equal to or greater than the image display region of the TN LCD panel. Polarizing plates are provided before and behind the TN LCD panel. The polarizing plate interposed between the diffusion plate and the TN LCD panel (the polarizing plate referred to as the "polarizer" hereinafter) has a function of converting the light from the surface light source into a linearly polarized light. The analyzer plate interposed between the TN liquid crystal panel and an observer of the display screen (referred to as "analyzer" hereinafter) has a function of interrupting the light according to the degree of modulation of the light incident on the TN LCD panel. Normally, the polarizer and the analyzer are arranged so that the polarization directions of the two cross each other at right angles.
The surface light source is formed as described above, and the light from the surface light source is converted into a linearly polarized light by the polarizer. The TN LCD panel modulates the linearly polarized light according to an applied video signal. The analyzer shields the light or allow the light to be transmitted according to the degree of modulation. Thus, a display image is formed. The display image can be viewed through magnification by a magnifying lens arranged between the analyzer and the observer.
As apparent from the above description, it is necessary to use linearly polarized light in a display device employing a conventional TN LCD panel. Therefore, totally two polarizing plates are required to be arranged on the light input side and light output side of the LCD panel. Since the polarizing plates absorb not less than half of light, which results in a problem that only a low-luminance display image can be obtained.
For the same reasons, no high-luminance image display can be performed even when constructing an LCD image projection type television system by using the conventional display device. Furthermore, the polarizing plates absorb light to generate heat, and the generated heat is conducted to the LCD panel to consequently deteriorate the panel. The polarizing plates themselves deteriorate due to light absorption to result in worsening the degree of polarization.
Any video camera is required to be compact and light in weight in terms of its portability and operability. Therefore, currently there is a growing trend of introducing a liquid crystal panel as a display system for a view finder. However, any view finder employing an LCD panel consumes fairly much power in the current situation. For instance, the power consumption a view finder employing an LCD panel amounts up to 1.1 W including the LCD panel power consumption of about 0.1 W and the light source power consumption of about 1.0 W. Any video camera has a limited battery capacity to assure its compactness and light weight. When a great power is consumed by the view finder, there is a serious problem that only a short continuous operation time is permitted.
The reason why the view finder consumes much power is that the polarizing plate to be used for the TN LCD panel has a total transmittance of only about 30% assuring a low utilization efficiency of light. In addition, a light box comprised of the fluorescent tube and the reflection plate is required to form a surface light source having less unevenness in luminance. Therefore, a diffusion plate is provided between the TN LCD panel and the fluorescent tube. When a diffusion plate having a low light diffusion capability is used, a lighting pattern of the fluorescent tubes appears to be visible through the display screen to consequently degrade the display image quality. Therefore, a diffusion plate having a high light diffusion capability is used. However, when the degree of light diffusion is increased, generally the light transmittance of the diffusion plate is reduced. Therefore, it is the last resort to increase the output quantity of light from the light source in order to obtain necessary luminance, which also results in increasing the power consumption of the light source.
Furthermore, in the conventional display device, one drive circuit has been outputting a signal in positive polarity and a signal in negative polarity. When a greater number of pixels are included in the display device, a higher operation frequency of the drive circuit is required. It is noted that the drive circuit mentioned herein means a drive IC for outputting a signal to each signal line of the display device. When the operation frequency is made higher, the output signal cannot be increased in amplitude. According to the existing techniques, when the operation frequency is about 20 MHz, it is permissible to achieve a signal amplitude of about .+-.6 V or smaller, meaning that a signal amplitude up to a voltage slightly lower than 12 V can be achieved in design. It is difficult to design a drive circuit IC capable of yielding a voltage higher than the above-mentioned voltage in regard to its endurance voltage and heat resistance. The scheme of increasing the signal amplitude also results in increasing the heat generation of the IC to consequently cause a faulty operation or the damage of the IC. The scheme of increasing the operation frequency also causes heat generation. Increasing the signal amplitude requires increase of the scale of an output buffer element of the IC, which also results in dimensional increase of the IC and cost increase. Furthermore, introduction of a special process for producing such an IC is required.
In order to decrease the operation frequency of the drive circuit, there is a method of dividing the display region into several segment regions, providing a drive circuit for each segment region, and making them operate in parallel. However, the above-mentioned method has the drawback of a difference in level between the signals input to the segment regions to disadvantageously result in the presence of junctures between the segment regions. There is currently a trend for increasing the pixels in amount in excess of one million pixels to add more difficulties to designing and developing an IC for the drive circuit in terms of its endurance voltage and heat resistance.
The display device employing the polymer dispersion liquid crystals requires no polarizing plate. Therefore, a high luminance display can be achieved, however, it has a drawback of low contrast. In order to increase the contrast, it is necessary to improve the light diffusion capability in a condition where no voltage is applied. In order to improve the light diffusion capability, it is required to increase the liquid crystal film thickness. However, when the liquid crystal film thickness is increased, the voltage for making the liquid crystals transmissive is higher.